1. Field of the Invention
This invention relates in general to a cold cathode fluorescent lamp device, and in particular, to a high luminance, high efficiency, long lifetime monochromatic, multi-color or full-color cold cathode fluorescent lamp display (CFD). The invention is particularly useful for use in illumination and for ultralarge screen display device for displaying character, graphic and video image, and for displaying traffic information, for both indoor and outdoor applications.
2. Description of the Prior Art
Hot cathode fluorescent lamps (HCFLs) have been used for illumination. The HCFL operates in the arc gas discharge region. It operates at a relatively low voltage (of the order of 100 volts), large current (in the range of 60 milliamps), high efficiency (such as 80 lm/W), and the cathode is usually operated at a relatively high temperature such as 400 C. Typically, the cathodes would first need to be heated to an elevated temperature by means of a starter and a ballast before the HCFL may be turned on and operated at its optimum temperature. Thus, in order to turn on an HCFL, a voltage is applied to the starter which generates gas discharge. The heat produced by the gas discharge heats up the cathode and an electron emission layer on the cathode to an elevated temperature so that the layer emits electrons to maintain the gas discharge. The gas discharge generates ultraviolet radiation which causes a phosphor layer in the lamp to emit light.
When the cathode and the electron emission layer are first heated to an elevated temperature during starting, the heating causes a portion of the electron emission layer to evaporate, so that after the HCFL has been started a number of times, the electron emission layer may become deficient for the purpose of generating electrons, so that the HCFL needs to be replaced. This problem is particularly acute for displaying information that requires constant starting and turning off the HCFLs. Thus, HCFLs are not practical for use in computer, video, and television applications. For the purpose of illumination, HCFLs requires starters and ballasts, which may also become defective after a period of constant use. This also reduces the lifetime of the HCFL. It is thus desirable to provide an illumination device with improved characteristics.
Currently available traffic light and outdoor large size sign displays are normally made of incandescent lamps. They have high brightness, but many drawbacks:
a. High maintenance cost because of short lifetime and low reliability. This is the case especially for traffic lights or signs on free ways, where changing and repair of the lights are very inconvenient and expensive. PA1 b. High power consumption because of low luminous efficiency, which is about 10 lm/W. For traffic lights and other multi-colored displays, luminance efficiency is even lower because colored light is obtained by filtering white light emitted from the incandescent lamps, so that the colored light so obtained is much reduced in intensity. The effective efficiency for such applications is only 4 lm/W or lower. PA1 c. Under direct sunlight, ON/OFF contrast is very low, i.e., even OFF status looks like ON, which can cause fatal results.
It is, therefore, desirable to provide an improved illumination device which avoids the above-described disadvantages.
A plasma display panel (PDP) type device operates in the gas discharge plasma region. Unlike the HCFL, the electrodes are located not inside the glass tube but outside. As a whole, the plasma region of the tube is electrically neutral. The glass tube typically contains no mercury and contains only an inert gas such as xenon to generate ultraviolet light. The PDP has very low efficiency, usually at about less 1 lm/W. For this reason, PDP a type device is generally not used for illumination at all and is used only for displays.
The major prior technologies for ultra-large screen display are as follows:
A. Incandescent Lamp Display
The display screen consists of a lot of incandescent lamps. The white lamps are always used for displaying the white and black characters and graphics. The color incandescent lamps, which use red, green, and blue (R, G, B) color glass bubbles, are used for displaying multi-color or full-color characters, graphics and images. The incandescent lamp display has been widely used for outdoor character and graphic displays and possesses certain advantages such as low cost of lamps. Nevertheless, this technology suffers from the following disadvantages: low luminous efficiency (i.e., the efficiency of white lamps being about 10 lm/W; and that of lamps emitting R, G, B light being less than one-third that of white lamps); high power consumption; poor reliability, unexpected lamp failure; short lifetime; expensive maintenance cost; long response time and unsuitable for video display.
B. Light Emitting Diodes (LED)
LED has been widely used for indoor large screen and ultra-large screen display, to display multi-color and full-color character, graphic and video images. This display is able to generate high luminance for indoor applications and can maintain a long operation lifetime at indoor display luminance level. The disadvantages of LED, however, are as follows: low luminous efficiency and high power consumption especially for the ultra-large screen display; low luminance for outdoor application especially the wide viewing angle is required or at direct sunlight; expensive, especially for ultra-large screen display because the need of a lot of LEDs; and lower lifetime at high luminance level.
C. Cathode Ray Tube (CRT)
CRT includes Flood-Beam CRT (e.g., Japan Display '92, p. 385, 1992), and matrix flat CRT (e.g., Sony's Jumbotron as disclosed in U.S. Pat. No. 5,191,259) and Mitsubishi's matrix flat CRT (e.g. SID '89 Digest, p. 102, 1989). The CRT display is generally known for its ability to produce good color compatible with color CRT. The disadvantages of CRT are as follows: low luminance for outdoor applications; low contrast at high ambient illumination operating condition; short lifetime at high luminance operating condition; expensive display device due to complex structure and high anode voltage about 10 kv.
D. Hot Cathode Fluorescent Display
Hot cathode fluorescent technology has been used in a display system called "Skypix" (SED '91 Digest, p. 577, 1991) which is able to generate high luminance at about 5000 cd/m.sup.2 so that it may have adequate brightness in direct sunlight. The disadvantages of this system are: low luminous efficiency due to hot cathode and short gas discharge arc length; very high power consumption and short lifetime because a hot cathode display requires too many switchings in a video display.
At present, the incandescent lamps are commonly used for outdoor character and graphic displays.
The flat matrix CRT, including flood beam CRT and matrix CRT, is the most common display for outdoor video display. Neither of these two technologies presents a display system which can be used in both indoor and outdoor applications possessing unique features overcoming all or substantially all of the disadvantages described above.